High voltage cable termination

ABSTRACT

A cable termination is provided with a terminator having a rigid insulator suited for outdoor application and an inner stress relief element contained in the insulator. The terminator is applied in telescopic relation to a prepared end of a power cable which has a region of high electrical stress. The inner stress relief element includes a compressible portion having electrically conductive properties and forming a stress control shield, and a contiguous high dielectric strength portion. The two portions are related to one another such that the compressible portion provides continuous compliance of the stress relief element on the prepared cable end of any solid plastic or elastomeric insulated cable that is within a given range of cable sizes. The terminator may include a sealable compression lug for the making of an external electrical connection to the terminator.

United States Patent 1191 Lusk [ HIGH VOLTAGE CABLE TERMINATION [75]Inventor: George E. Lusk, Downers Grove, Ill.

[73] Assignee: G & W Electric Specialty Company,

Blue Island, 111.

[22] Filed: Oct. 6, 1972 [2]] App]. No.: 295,507

[52] US. Cl. 174/73 R, 174/75 D [51] Int. Cl H02g 15/02 [58] Field ofSearch 174/19, 20, 73 R, 73 S,

174/74 R, 75 R, 75 D, 75 F, 78, 80, 142; 85/1 C; 339/265 R, 265 F, 272 R[56] References Cited UNITED STATES PATENTS 2,728,810 12/1955 Ziehr174/75 R 3,290,428 12/1966 Yonkers 174/73 R 3,355,541 11/1967Hornbergen. 174/73 R 3,404,211 10/1968 Nicholson 174/73 R X 3,494,2432/1970 Kleinhenn.... 85/1 C 3,548,070 12/1970 Duenke 174/73 R x3,585,274 6/1971 Tomaszewski et al. 174/73 R 3,634,604 l/l972 Lusk174/73 R 3,662,082 5/1972 Heppner 174/73 R X 3,715,449 2/1973 Cunninghamet al 174/73 R x OTHER PUBLICATIONS Anderson Electric Corp.Manufacturers Catalog, No.

1451 Mar. 12, 1974 57, Section B, page 34-B, Oct. 1, 1957,

Primary ExaminerLaramie E. Askin Attorney, Agent, or Firm-Fitch, Even,Tabin &

Luedeka [5 7] ABSTRACT A cable termination is provided with a terminatorhaving a rigid insulator suited for outdoor application and an innerstress relief element contained in the insulator. The terminator isapplied in telescopic relation to a prepared end of a power cable whichhas a region of high electrical stress. The inner stress relief elementincludes a compressible portion having electrically conductiveproperties and forming a stress control shield, and a contiguous highdielectric strength portion. The two portions are related to one anothersuch that the compressible portion provides continuous compliance of thestress relief element on the prepared cable end of any solid plastic orelastomeric insulated cable that is within a given range of cable sizes.The terminator may include a scalable compression lug for the making ofan external electrical connection to the terminator.

21 Claims, 10 Drawing Figures ill PAIENTEUIAR 12 I974 3'796' 821 SHEET 2OF 3 HIGH VOLTAGE CABLE TERMINATION The present invention relates tohigh voltage cable terminations, and more particularly to terminatorsand the stress control elements that are used to fill the space betweenthe terminator insulators and their respective cables.

The term terminator has been commonly used in industry interchangeablywith cable termination, terminal or pothead. The term cable terminationis used throughout this application to cover the complete ass'embly of acable, riser, support structure, etc., as it is when ready for service.The term terminator or pothead is used to refer to the device andmaterials normally supplied by a manufacturer to terminate a cable inthe field. An Institute of Electrical and Electronic Engineers (IEEE)publication 48 Standard For Potheads, issued May, 1962, generally coversthe terminology for these devices.

As used herein, the term high voltage refers to a rating of at least15,000 volts kv). Cables having such a voltage rating generally containan electrically conductive layer (shielding) over the dielectricinsulation layer. This shield layer generally consists of a carbonfilled plastic or elastomer directly extruded over the insulationproper. Metal wires or tapes may be wound in a helical formconcentrically around the cables on the shield layer to enhance thecurrent carrying capacity of the shield system. The shield system isgrounded. The potential of the conductor within the cable may beconsidered at one hundred percent of the operating voltage of thesystem; and the potential of the grounded shield at zero. The voltagegradient, i.e., the difference in potential per unit thickness of theinsulating medium, between the conductor and the shield may berepresented by an infinite number of concentric cylindrical surfaceswithin the cable insulation, each surface being of the same voltagepotential. In a longitudinal section, these cylindrical surfaces wouldthen be seen as equipotential lines. The voltage gradient represented bythe lines is the greatest close to the conductor and the least away fromthe conductor.

When a power cable is terminated, such as for making a connection toelectrical equipment, the conductor is exposed by removing some lengthsof both the insulation and the shield layer. The shield layer is removedfrom a length ofinsulation so as to separate the exposed conductor andthe grounded shield and thus provide adequate creepage distance from thelive conductor to ground; i.e., the end of the grounded shield, which isat zero potential, isseparated from the exposed conductor, which is atone hundred percent potential. This separation is made by a distanceadequate to avoid flash over on the surface of the insulationtherebetween.

Such a termination creates an abrupt discontinuity in the electricalcharacteristics of the cable. Further, it exposes the cable insulationto ambient atmosphere, which contains moisture, gases, and particulatematter. The baring of the conductor exposes it to corrosion, and thediscontinuing of the cable shield materially increases the maximumvoltage gradient (volts/mil) of the insulation in the area of the cableshield end and immediately changes the shape of the resulting electricalfield to introduce high longitudinal voltage gradients along the surfaceof the insulation adjacent the cable shield end. Thus, the maximumvoltage gradient is shifted from a radial stress, which diminishesoutwardly from the conductor, to a longitudinal stress at the end of thecable shield layer. The nature 'of the cable insulation is such that itmore readily withstands the stress in the radial direction than along alongitudinal surface or interface, and risk of breakdown is thereforegreater in the longitudinal direction.

One purpose of a terminator in a cable termination is to compensate forthis shift in the electrical field and electrical stress characteristicsat the cable terminal. Another purpose is to protect the cable endportions from the effects of ambient elements.

Terminators may be of the so-called wet type or of the so-called drytype. They generally include a rigid insulator shell or body whichelectrically insulates and protects the cable termination against theeffects of wind, precipitation, ultraviolet radiation, and air-bornecontamination.

Typically in the wet type, the insulator body contains, for example, astress control shield applied at the termination of the cable shieldlayer and a suitable dielectric compound or fluid that fills the spaceor cavity between the cable and the inside wall of the insulator body.The stress control shield may be of metal or of hand-appliedelectrically conductive tape that is built up in a suitable form.

Typically in the dry" type, the insulator body contains, for example, amolded stress relief element of a noncompressible elastomer which has aninside diameter that provides an interference fit on the insulationsurface of a specific size of cable. In such instance, the material ofthe molded stress relief element is also the dielectric medium thatfills the aforementioned space or cavity. Where dielectric strength isneeded, the space is filled with a dielectric material to exclude air,which is generally not suitable for such a medium.

Some disadvantages attend the foregoing terminators when used on theextruded insulation cables. For example, the use of a dielectriccompound or fluid in the terminators is inconvenient to handle in thefield, and further, the compound may be workable only at certaintemperatures; the hand applied stress control shield involves both skillin construction and considerable time in installation; and thenoncompressible molded stress relief element usually requires arelatively high force to slide it over the cable end because oftheinterference fit, and it is suitable for application only to a givennominal size of cable. Even here a difficulty may be encountered becauseof the variation in diameter during a cable run as well as between runsin manufacturing and a particular noncompressible element may notprovide an interference fit on all cable segments of a given nominalsize of cable.

US. Pat. No. 3,634,604 issued Jan. 11, 1972 to George E. Lusk, disclosesan improved power cable terminator employing a dry type, molded stressrelief element made of a compressible dielectric material. A givenstress relief element of the type disclosed is capable of accommodatinga range of cable diameters. The present invention is an improvement ofthe type of cable termination therein disclosed, especially for use inhigher rated voltage systems than therein contemplated.

It is an object of the present invention to provide an improvedterminator for high voltage extruded insulated cable terminations.

It is another object of the present invention to provide an improvedterminator and an inner stress relief element therefor that is capableof accommodating a range of power cable diameters for a given elementsize in a high voltage system.

It is yet another object of the present invention to provide a dry typehigh voltage terminator that is capa ble of being easily installed inthe field.

These and other objects of the invention are more particularly set forthin the following detailed description and in the accompanying drawingsof which:

FIG. 1 is a side view in section ofa cable termination embodying variousfeatures of the present invention;

FIG. 2 is a side elevational view partly in section of a high voltageterminator for use in the termination of FIG. 1 and further including asealable compression connector at the hood end of the terminator;

FIG. 2a is an end view of the sealable compression connector of FIG. 2;

FIG. 2b is a fragmentary view of a portion of a sealable set screw ofthe sealable compression lug of FIG. 2 with dimensions exaggerated forclarity of illustration;

FIG. 3 is a perspective view of a stress relief element for use in thestructures of FIGS. 1 and 2;

FIG. 4 is a sectional view in perspective taken along the line 4-4 ofthe element of FIG. 3;

FIG. 5 is a sectional view taken along the line 55 of FIG. 4;

FIG. 6 is a sectional view of the termination taken along the line 66 ofFIG. 1 and illustrating various features of the present invention;

FIG. 7 is a diagrammatic view ofa cable end illustrating the intensitypattern of a typical electric field in a high voltage cable at the endof the cable shield; and

FIG. 8 is a diagrammatic view of a portion of the termination of FIG. 1depicting some effects of the illustrated embodiment on the electricfield pattern of FIG. 7.

Referring now to FIG. 1, the reference numeral 11 refers generally to ahigh voltage cable termination in accordance with a preferred embodimentof the present invention.

Very generally, the end ofa high voltage power cable 13 has a terminator15 applied thereon in telescopic relation. The terminator 15 includes arigid insulator body or housing 17 having a top cap 19 at its hood endand a base cap 20 at its base end, a mounting plate 21, and a stressrelief element 23, which fills the space intermediate portions of therigid insulator body 17 and portions of the end of the cable 13. Theelement 23 has a compressible portion 77 which is in the form of astress control shield. Contiguous to this shield portion 77 is a highdielectric strength portion 79, which allows use of the element in ahigh voltage system, and this in turn is contiguous to anothercompressible portion 81. As is described in detail hereinafter, theseportions are related to each other within the body of the element 23such that in spite of the high dielectric portion, the compressibleportions cause a continuous compliance of the entire element 23 to theend of the cable it surrounds. Such compliance eliminates the occurrenceof interfacial voids, which could lead to dielectric breakdown, and ismaintained even when the cable expands and contracts with temperaturechanges. At the same time, the element provides the terminator with anability to accommodate cables having a range of diameters.

The high voltage power cable 13 includes a conductor 25, dielectricinsulation 27 surrounding and encasing the conductor 25, a shield layer29 surrounding the dielectric insulation 27, and concentric neutral orground wires 31 wound around and in contact with the shield layer 29.The cable 13 may be prepared for receiving the terminator 15 by beingstripped to expose a length of the bare metal conductor 25 at the end ofthe cable. The shield layer 29 is removed from around a length of thedielectric insulation 27. The ground wires 31 are removed from a portionof the end of the shield layer 29 and are bound to form a common groundwire 32 for subsequent connection to the mounting plate 21. This leavesan exposed length of the shield 29 intermediate the binding of theground wires 31 and the exposed dielectric insulation 27. A lubricant,such as a silicone grease, is applied to the cable insulation and theshield. A protective plastic guide cap 33 is temporarily placed over theend of the cable conductor to protect internal components of theterminator 15 from being damaged by the metal conductor strand endsduring assembly.

The terminator 15 is then slipped over the prepared cable end until theexposed conductor25 extends beyond the terminator and the dielectricinsulation 27 shoulders against the top cap 19 of the terminator. Theprotective guide cap 33 is then removed, its purpose having beenfulfilled.

Referring now in more detail to the illustrated embodiment, porcelain isgenerally used for the rigid insulator body 17 because it is aself-cleaning, inorganic, homogeneous material which does not carbonizewhen subjected to leakage currents and is not subject toweathering,radiation, or chemical damage. Porcelain also has hightracking resistance, i.e., the ability of the material to resist theformation of a conductive path by an arc adjacent its surface.Additional materials, however, such as other ceramics, glass and epoxyresins, also may be suitable for this insulator body.

The top cap 19 and the base cap 20 may be made of metal, such asstainless steel, and both are firmly applied to the insulator body 17 asby rolling their edges over the ends of the body, as indicated by thereference numeral 35. Intermediate the ends of the insulator body andeach of the caps is a gasket 37 of a suitable elastomeric material. Thetop cap 19 has a clearance opening 38 for receiving the conductor 25,and the base cap 20 has a clearance opening surrounded by an annular lip39. These clearance holes preferably will pass the largest cable in thediameter range of cables a given terminator 15 is to accommodate,examples of which are provided hereinafter.

Physical support of the cable at the point of termination is provided bythe mounting plate 21, which is secured to the base cap 20 by suitablemeans, such as threaded studs 40 and complementary nuts 41. The groundwire 32 may be connected on any of these studs. Holes 42 are provided inthe mounting plate 21 for use in attaching the termination by suitablemeans to a separate supporting structure (not shown).

The stress relief element 23 is disposed within the rigid insulator body17 such that its internal diametral surface is in direct contact withportions of the prepared cable end when mounted thereon. As is describedin detail hereinafter, there is sufficient pressure at the interface ofthese surfaces to form a seal against the entrance of atmosphericconditions at the base end of the terminator 15.

Turning now to FIG. 2, at the top cap 19, or hood end of the terminator15, a protective seal is formed by a sealing gasket 43 partiallyembedded in a gasket groove 45 in the base of an attachable sealableconnector, such as a hood lug 47. The illustrated hood lug 47 is aspecially modified so-called aerial lug useful in joining the conductor25 to electrical apparatus or bus or to another cable. The sealable hoodlug is attachable to the terminator by means of threaded studs 49suitably affixed to the top cap 19 (FIG. 1). In FIG. 2a, three mountingslots 50 are shown in the base of the hood lug 47. These slots receivethe studs 49 for mounting the hood lug to the top cap 19 of theterminator. The lug is capable of accepting a range of cable sizes, bothinternally and externally, and the slots provide a selfcentering featurewhich is useful in accommodating the cables on which the terminator maybe applied. Generally, the cables for the external connection areconsistent in size with those of the terminator.

When the hood lug 47 is securely mounted on the top cap 19, such as bynuts 51 on the threaded studs 49, the sealing gasket 43 forms a sealbetween the surfaces of the base of the lug 47 and the top cap 19. Abore 53 in the lug receives the bared conductor 25, which may bestranded. In the illustrated lug 47, suitable threaded openings 54 arein communication with the bore 53 and receive set screws 55 to form acompression fit of the conductor 25 in the bore 53 of the lug. Thescrews 55 are turned inwardly against the strands of the conductor tosomewhat spread the strands and cause them to be compressed against andconform to the inner walls of the bore 53. Although three set screws 55are illustrated, it is understood that the invention is not intended tobe limited by a specific number of set screws. There could be more orless. The compression fit feature allows the illustrated lug 47 toreplace the crimp type lug and a crimping tool therefor typically usedfor this type connection. Thus, this lug contributes to simplifying thefield installation of the terminator.

As may be seen in FIG. 2b, the set screws are selfsealing, preferably asby a suitable plastic coating 58 on their threaded surfaces to seal outthe environmental elements from the strands of the conductor 25. Anannular tapered surface or countersink 60 (FIG. 2a) is provided at theouter end of each of the threaded openings to ease the entrance of thecoated set screw 55 therein so that the coating 58 will not be peeledoff by a sharp starting thread of the opening. Alternatively, theplastic coating could be applied on the threads of the opening. The bore53 is completely closed except for the cable entrance in the base andthe three set screw openings.

Since the applied set screws seal their openings, the gasket 43 sealsthe interface of the base of the lug 47 and the top cap 19, and thegasket 37 seals the interface of the top cap 19 and the body 17, theenvironmental elements are prevented from entering the terminator 15from the hood end. As stated previously, the base end of the terminatoris sealed by the stress relief element 23. Thus, the dielectricinsulation 27 and the bared conductor 25 are completely contained withinthe terminator in a manner that makes them impervious to externalinfluences.

The illustrated hood lug 47 is preferably a casting, such as an aluminumcasting. A protuberant part of the casting is in the form of a clampbase 48. In the clamp base is a surface 62, which is in the form of ahalf cylinder whose axis is parallel to the general axis of theterminator 15, and a surface 64, which also is in the form of a halfcylinder but whose axis is transverse to the general axis of theterminator 15. The radii of the two semicylindrical surfaces may be thesame. The purpose of these surfaces is to establish an externalelectrical connection by receiving a suitable conductor from externalcircuitry or equipment, such as a transformer lead (not shown). Eithersurface (62 or 64) may be used as is convenient under the circumstancessurrounding the connection, i.e., straight on with the terminator or atright angles to it.

The external electrical connection is completed by drawing up securelyagainst the external conductor a cooperating clamp 48a by suitablemeans, such as four mounting bolts 52 in suitable threaded holes in thebase 48. The clamp 48a has two semicylindrical surfaces 66 and 68 atright angles to each other in one of its mounting faces, and it has twosemicylindrical surfaces 71 and 73 at right angles to each other in theother of its mounting faces. The four mounting bolts 52 are disposed atthe four corners of a square. Thus, any of the surfaces 66, 68, 71 or 73of the clamp 48a may be made to align with and oppose either of thesurfaces 62 or 64 in the base 48. In the illustrated embodiment, thesurfaces 66, 68, 71, 73 differ from one another in radius, thusproviding combinations of cooperating surfaces to accommodate varioussizes of a conductor. Alternatively, such combinations could be producedby surfaces of different radii in the base cooperating with one or moresurfaces in the clamp.

This structure of the hood lug 47 provides an important feature of thepresent embodiment. Not only is the lug capable of internally acceptingand sealing therein a range in size of cables by the sealable set screws55 providing a compression fit on a conductor in the bore 53, but alsothe lug is capable of electrically joining thereto, without additionalparts, external conductors that may be consistent in size to theinternal cables. Further, other aerial lugs having a like base but adifferent cable size range are interchangeable on the top cap 19.

As shown in FIG. 2, the outside surface 56 of the rigid insulator body17 is finned or corrugated in a conventional manner to increase thecreepage distance along the surface between the top cap 19 (one hundredpercent potential) and the base cap 20 (zero potential). The innersurface of the rigid insulator body 17 is smooth and includes a surface57 formed by a general axial bore through the body and a surface 59formed by a counterbore. The counterbore is of a larger diameter thanthe general bore and contains the stress relief element 23.

The stress relief element 23 is in the general form of a cylinder. Theouter surface 61 of the element 23 is essentially equal to the diameterof the inner surface 59 in the base end of the insulator body 17 andthus conforms to and contacts this inner surface when the stress reliefelement is installed in the insulator body. The element 23 has a generalaxial bore through it which forms a general inner surface 63. It isnoted that when the element 23 is installed in the body 17, the innersurfaces 57 of the body and 63 of the cone are axially aligned, but thediameter of the inner surface 63 is less than the diameter of the innersurface 57.

Returning now to FIG. 1, it is the general inner surface 63 of thestress relief element 23 that is in contact with a portion of thedielectric insulation 27 of the cable 13. A counterbore extends axiallypart way into the stress relief element 23 at its base end to form aninner surface 65, which is axially aligned with, but of a largerdiameter than, the general inner surface 63 of the stress reliefelement. A chamfer 67 adjoins the inner surface 65 to facilitateassembly of the terminator. The inner surface 63 surrounds an endportion of the shield layer 29, which terminates at 69. This terminal 69is at the end ofa taper 70 made at the end of the shield 29 to bringabout a more uniform wall thickness around the insulation at this point.This is especially important where the shield 29 is applied to the cableby extrusion and may not be of a completely uniform thickness around thecircumference of the cable.

It will be noted that the disposition of the stress relief element 23within the insulator body 17 and the disposition of the completeterminator on the prepared end of the cable 13 are such that the offsetbetween the inner surfaces 63 and 65 of the element 23 abuts theterminal 69 of the shield 29. The diameter of the annular inner surface63 is slightly less than the diameter of the shield layer, and thisforms a close fitting, intimate contact between the two surfaces. Suchcontact excludes air, moisture and other ambient elements. As isdescribed in detail hereinafter, this is also an electrical contact thatparticipates in the dielectric stress relief in the cable insulationadjacent the terminal 69 of the shield.

The above described disposition of the inner surface 65 of the stressrelief element 23 cooperates with the earlier mentioned cablepreparation in which the cable may be stripped or otherwise initiallyprepared to expose lengths of the conductor 25, the dielectricinsulation 27, and the shield layer 29. These lengths are a function ofthe creep strength of the total termination and the voltage of thesystem on which it is employed. An example of one specific constructionof the preferred embodiment has been built for a system of approximately35 kv, in which the termination utilized substantially the followingapproximate dimensions: length of the terminator 15 less the hood lug4'7, 14 inches; length of the bared conductor 25, 3% inches; length ofthe bared dielectric insulation 27, 12 inches; length of the baredshield layer 29, 5 inches; and length of the stress relief element 23, 7inches.

Having observed the details of the disposition of the stress reliefelement 23 in its interposed relation between at least portions of therigid body insulator 17 and the prepared end of the cable 13, attentionmay now be given to the structure of the element 23 itself, the specificrelation of the element to other parts of the cable termination, and theattendant advantages of both. The element 23 may be an assembledstructural composite of separate portions having definedcharacteristics, or it may be formed in a continuous unit with a lessabrupt change between the portions of defined characteristics. Forpurposes of illustration, the assembled composite is shown and describedand is best seen in FIGS. 3-5. It is understood, however, that theinvention is not limited to this composite construction.

Turning first to FIGS. 3 and 4, the illustrated stress relief element 23is seen to be of cylindrical form and to comprise three portions orsections. A first or base portion 77 is that portion of the compositeelement 23 which when installed within the rigid insulator body 17(FIG. 1) is near the base cap 20 of the terminator 15. This base portion77 preferably is made of a closed-cell sponge elastomer, described indetail hereinafter, and has an inner surface 63a, which is in line withthe general inner surface 63 of the completed element 23, and a taperedsurface adjoining the surface 63a. The taper of the surface 80 isdirected outwardly from the axis of the element 23 and away from itsbase end, as best seen in FIG. 4. The tapered surface 80 forms a cavityhaving a shape closely resembling that of a truncated cone. Extendinginwardly in the base portion 77 from its base end is the counterborehaving the surface 65 and terminating at a shoulder 83. The chamfer 67is at the leading edge of this counterbore.

All inner and outer surfaces of the base portion 77 are caused to beelectrically conductive in a suitable manner, such as by covering thesurfaces with a layer 78 of a conductive coating substance. Suchconductive surfaces cause the base to be a stress control shield. Onesuitable substance for forming the conductive coating is manufactured byGeneral Electric Company and is identified as General ElectricSemi-Conducting Silicone Resin SR 531. An alternative is to make theentire base portion 77 conductive by insertion of electricallyconductive carbon particles within the walls of the sponge material.

Another part of the composite element 23 is a second portion 79. Thissecond portion has an outer surface at its base end of a shape thatcomplements and interfits with the cavity in the base portion 77 whenthe two are axially aligned. This second portion preferably is made of aclosed-cell elastomer of a higher density with a smaller average cellsize than that of the base portion 77 to provide higher dielectricstrength in the zone of the element occupied by this second portion. Thedielectric strength of the body material constituting the aforementionedfirst portion 77 is relatively unimportant in view of its conductivesurface. Absent the conductive surface, however, the sponge materialdoes have an inherent dielectric strength, and this second portion 79when compared therewith is, of course, of higher dielectric strength.The higher density and smaller average cell size of this portion givesit a nature approaching that of solid material. Alternatively, thematerial may be a solid elastomer. This second portion has an angular orgenerally frustoconical form, at least at its base end, and preferablyhas a tapering cavity in its other end for receiving yet another part ofthe cone. This other part, or third portion 81 of the composite element23, has an outer surface at its base end that is shaped to complementand interfit with the cavity of the second portion 79 when the twoportions are axially aligned. This portion 81 may be of the sameclosed-cell sponge elastomer as the base portion 77, but it is not madeelectrically conductive. Its density is lower (larger cell size) thanthat of section 79 providing a graded density stress element system, thehigher density zones for the higher voltage gradient areas, as will bedescribed in more detail hereinafter. The third portion 81 completes thegeneral cylindrical form of the element.

The composite element 23 may be completed by assembling the unitportions axially in their interfitting relation and bonding the sectionstogether with a suitable bonding agent. Alternatively, the cone 23 couldbe continuously molded in one piece with the aforementioned members ofthe cone being zones of defined characteristics.

It is noted that in the composite element 23 there is selectivepositioning of the density distribution of the materials within the bodyof the element. FIG. illustrates a section taken arbitrarily through thesecond portion 79. According to the illustrated embodiment, thedistribution of the solid material is within a zone of the element 23that surrounds especially a critical high electric stress region 82(FIG. 1) of the cable insulation 27 adjacent the end 69 of the cableshield 29. This distribution is such that any plane passing through thecylindrical element normal to the axis of the element and particularlyWithin the region 82 will not pass through solid material only. There issponge material adjacent the higher density material that any such planewould intersect. It will be noted that the relative disposition withinthe element of particularly the first two portions is such that theparts of the high dielectric portion intersected by the plane passingthrough the high stress region 82 are disposed inwardly of thecompressible portion and surround the region, whereas the parts of thecompressible portion intersected by the plane are disposed outwardly ofthe high dielectric portion and, as seen in FIG. 1, are in contact withthe rigid insulator body 17. This structural combination provides animportant advantage of combining the dielectric properties of the higherdensity material in this zone of the element with the mechanicallycompressible characteris tic of cellular material, and this achievescontinuous compliance of the element 23 to the vable to eliminate voidsat the interface between the inner diametral surface of the element andthe cable which could lead to dielectric breakdown, particularly in theregion 82 of high electrical stress. This compliance is achieved eventhough solid, high dielectric strength material is incorporated in thebody of the element 23, and it is achieved in a manner described below.

The outer diameter of the composite stress relief element 23 isessentially the same as the inner diameter of the counterbore of therigid insulator body 17 (FIG. 1) so that the inner surface 59 of theinsulator body and the outer surface 61 of the composite element are incontact with each other when the assembled terminator is slipped intelescopic relation over the prepared end of the cable 13. Because theinsulator body 17 is rigid, it confines the composite stress reliefelement 23 and holds it under radial compressive stress when the elementis interposed between the insulator body and the cable, as illustratedin FIG. 6. The arrows depict the direction of both the initial andreactive forces, i.e., radially outwardly to the rigid body 17 andradially inwardly to the cable. These forces react in the same manner onall surfaces of the exposed portions of the prepared cable end that aresurrounded by the composite stress relief element 23 and the rigidinsulator body 17. In the zone of the element 23 surroundingparticularly the region 82 of high electrical stress of the cable, theseradial forces, like the aforementioned plane, in tersect parts of boththe high dielectric strength portion 79 and the compressible portion 77of the element, and the compressible portion assures the continuouscompliance of this zone of the element to the cable withoutobjectionable sized voids.

The effect of disposing the conductive portion 77 of the stress reliefelement 23 in gripping contact with the end 69 and adjacent portion ofthe conductive shield 29 is best seen in a comparison between FIGS. 7and 8. Referring first to FIG. 7, typically the dielectrical stressconditions adjacent the end of the shield 29 are extremely severe whencompared to those within the run of the cable 13 under the shield layer29. For example, electrostatic flux lines 85 are radial within the runof the cable 13, and equipotential lines 87 are parallel within the runof the cable 'where surrounded by the shield 29. Beyond the end 69 ofthe conductive shield, however, the flux lines 85 converge into a denseconcentration at 69, the result of which is high electrical stress in asmall area; and the equipotential lines 87 deflect out and around toemerge from the cable insulation 27 relatively close together, theresult of which is a region of high voltage longitudinal gradients atthis point in the insulation. In this regard, note particularly sampleequipotential lines A, B, C, and D and let A represent one-eighth fullvoltage, B represent one-quarter full voltage, C represent one-half fullvoltage, and D represent three-quarters full voltage. It is seen thatmost of the voltage is impressed across a very short length of the cableinsulation layer 27 longitudinally adjacent the end 69 of the conductiveshield 29. Such high longitudinal electrical stress in a small area maycause insulation failure. It is such a region represented at 82 in FIG.1.

Now referring to FIG.'8, the advantageous application of the illustratedembodiment of the terminator 15 with the inner stress element 23 isillustrated. The base portion 77 of the element, which has theconductive coating 78 therearound, surrounds the end portion of theshield 29 in a gripping relation, The conductive coating 78 is groundedthrough its intimate association with the cable shield 29, which isgrounded by the ground wires 31 (FIG. 1 As this base or stress controlshield portion of the stress cone tapers outwardly from the insulationlayer 27 of the cable, the electric field expands, as noted by the fluxlines 85, and the voltage gradients do not concentrate longitudinally,as noted by the gradual bending outwardly of the equipotential lines A,B, C, and D along the tapered surface 80 in the region 82 of highelectric stress.

The tapered surface 80 terminates in a rounded tip 89 before reachingthe general outer surface 61 of the stress relief element. Thisstructural feature of the cone shield termination reduces at this areathe intensity of voltage gradients that otherwise would occur in amanner similar to that depicted by lines A and B of FIG. 7. Whateverelectrical stress is built up in this area of stress control shieldtermination is contained without rupture by the high dielectric strengthportion 79 immediately adjacent this portion 77.

It will be noted that the stress relief element 23 does not fill thespace between the cable insulation 27 and the insulator body 17 alongthe entire length inside the insulator body. There is a space 91 (FIGS.1 and 8) adjacent the inner surface 57 of the body. Entrapped air is notexcluded from this space since the electric field (FIG. 8) followsgenerally outwardly under the stress cone shield or base portion 77 andonly passes through the space 91 at relatively low voltage gradientswhich do not cause dielectric breakdown of the air. Further, the ambientelements are effectively prevented from entering this space because ofthe seals at both ends of the'terminator 15. Thus, any part of the cableadjacent the space 91 is adequately protected without filling this spacewith a dielectric medium. This cavity could, of

lll

course, be filled with suitable dielectric sponge if so desired.

As a result of the foregoing combination structure of insulator body andinner stress relief element, the illustrated terminator decreases theintensity or concentration of the flux lines 85 and the equipotentiallines 87 in the high electric stress region 82 of the high voltage cable13, thereby increasing the voltage at which a given power cable may beused in a system. i The closed-cell elastomeric sponge material that isused for the preformed inner stress relief element 23 in the preferredembodiment is both compressible and compliable, The method of producingthe element 23 may utilize molding in a manner well known in the art.During the production of the element, voids, i.e., the result ofmaterial contamination or process irregularities, are prevented fromoccurring, and the size and distribution of the gas cells are selectedand controlled to obtain desired electrical and mechanical properties.Thus, the closed cells, rather than being voids, are purposely formedunder controlled process conditions. This cellular structure in asuitable elastomer in accordance with the present invention provides astress relief element that is capable of withstanding operational orcontingency stresses, whether electrical, thermal, or physical, that aretypical in a dielectric system.

Suitable elastomers for the illustrated stress relief element 23 includesilicones, ethylene-propylene, fluorosilicones, and fluorinatedelastomeric copolymers. An example of a fluorinated copolymer is Viton,a trademark of E. l. duPont de Nemours & Company.

A satisfactory cellular structure for the base portion 77 of the stressrelief element 23 when made of silicone rubber is a density of 0.022pounds per cubic inch with a maximum cell diameter of from 0.15millimeters to 0.75 millimeters. This same material and cellularstructure, as mentioned previously, may also be used for the thirdportion81 of the stress relief element when such third portion isemployed. A satisfactory cellular structure for the second, or highdielectric,portion 79 of the stress relief element 23 when made ofsilicone rubber is a density of 0.044 pounds per cubic inch with amaximum cell diameter of from 0.07 millimeters to 0.09 millimeters.

In the foregoing material description, it will be noted that bothmaterial density and cell size are variables. For the second portion,the preferred material is of smaller average cell size than that of thefirst portion and the density is greater. Dielectric strength isincreased in a closed-cell elastomer as the cell size is decreased,since the dielectric strength of a gas filled space is an inversefunction of the diameter of the cell. Accordingly, if the elastomer is asolid material, the dielectric strength is even higher, In such highdielectric strength material, however, the physical or mechanicalproperties of compressibility and compliability are substantiallydiminished.

The preferred embodiment, as indicated previously, employs a combinationof sponge material and essentially solid material in a continuousaxially aligned relationship that utilizes the advantages of both typesof material. The selective density distribution along the interface,i.e., along the tapered surface 80 between the first or base portion 77and the second portion 79, provides the present embodiment with a zoneof high dielectric insulation immediately adjacent the stress controlshield, or base portion 77, and this high insulation zone is insurrounding relation to the critical high electric stress region 82 ofthe prepared cable end when the terminator i5 is applied to the cable.

At the same time, the sponge material of the base portion is outwardlyadjacent the solid material in the zone of the element surrounding thecritical region 82 of the cable and provides compressibility in thiszone.

This structure of the stress relief element 23 in combination with itsrelation to the rigid body 17 in the present embodiment of theterminator 15 produces many advantages over known terminators. Amongthese advantages are the capability of the terminator of maintaining auniform, relatively constant, radial pressure at the interface of thestress relief element and the prepared cable end, especially in theregion of high electrical stress of the cable, so as to effect a properseal and good electrical contact in this region without voids on cablesof differing diameters, and there is no need for the addition of a highdielectric fluid or compound in the present structure, simplifying thefield installation of the terminator.

Further, the present structure of the element 23 causes it to yield whenthe cable expands and to follow when the cable contracts. During bothinstances, the present element 23 complies with the generallycylindrical surface of the portion of the cable which it surrounds, thusassuring contact at the interface even if the general cylindricalsurface is slightly out of form. During these volumetric changes of thecable, the radial compressive forces resulting from the presentstructural combination maintain the relatively constant pressure at theinterface sufficient to effect the aforementioned seal and electricalcontact. Yet the pressure is low enough to avoid excessive plasticdeformation (cold flow) of the cable dielectric insulation duringperiods of high temperature when the cable expansion is the greatest andthe resistance of the insulation to plastic defonnation is the least.Thus, the terminator is capable of operating effectively at temperatureshigher than the typical temperatures of previously known terminators. I

As mentioned previously, the present structural combination provides theterminator 1 .5 with the capability of accommodating cables of differingdiameters within a range of cable sizesv For example, in a 35 kv system,five sizes of the illustrated terminator 15 will accommodate powercables ranging in size from 1/0 AWG to 1,000 MCM. More specifically,each size will accomodate appropriately a 0.150 inch range of cablediameter. For each cable size accommodated, the above noted uniformradial pressure at the interface is effected. Further, this uniformradial pressure is maintained under anticipated ambient conditionsregardless of the current conducting state of the high voltage system onwhich the cable is employed.

The present stress relief element 23 as described herein has relaxationand creep modulii that are sufficient to compensate for the tendency ofthe typical elastomer to relax and to lose some gas from -its cells,where cells are present, and to provide an ability to effectivelyfunction in not only establishing a tight interface between the innerdiametral surface of the element on cables of differing diameters, butalso to maintain such during use.

Further, the element 23 as described has corona cutting resistance,ozone resistance, and low moisture and gas permeability. The closed gascells provide high ionization inception and extinction levels and resistdeleterious effects of ionization even during periods of over stress,such as occurs during lightning and abnormal switching surges. The ozoneresistance is important around the typical high voltage electricalsystems wherein appreciable amount of ozone may be generated aroundexposed high voltage electrodes. The low moisture and gas permeabilityare important where excessive absorption of moisture or a high rate ofcell gas loss would adversely affect the ionization inception andextinction levels associated with the gas space of the cells. Anadditional structural feature assists in minimizing moisture absorption.This feature is the relatively low exposed surface to volume ratio ofthe structure of the element 23. In this connection, note the exposedbase end 93 of the element 23 in FIG. 1.

summarising the advantages of the structure of the present stress reliefelement 23 and its interposed relation between the rigid insulator body17 and a cable 13 prepared end, the present element: (a) has the abilityto accommodate slightly out-of-shape cylindrical forms, such as thebared insulation of high voltage cables; (b) accomodates a diametralrange of these cables', (c) maintains sufficient radial pressure toeffectively form a seal and a good electrical contact at the interfaceof the element and the cables under both (a) and (b) even duringvolumetric changes of the cables as they respond to temperatureexcursions; (d) provides (a), (b) and (c) while preventing excessiveinternal pressures from developing during periods of high thermalconditions that could over stress the cable and cause plasticdeformation of the insulation; and (e) provides (d) and yet dos notallow inadequate pressure or space to develop at the interface of theelement and the cable whereby the effective seal against atmosphericelements and the electrical contact between parts would diminish or belost entirely.

Summarizing the advantages provided by the combination structure of theillustrated embodiment in the cable termination 11, including theforegoing stress relief element 23, the present terminator (a) enclosesthe cable end in an envelope having moisture resistant characteristics,(b) provides means to alleviate in crease in electrical stresses on thecable insulation caused by the interruption of the cable shield, (c)provides means in the form of a scalable compression lug for an externalelectrical connection to the terminator with conductors having currentcarrying capacity consistent with the cable conductor characteristics ofthe termination, (d) provides means for physical support of the cable,(e) is effective for use on a range of cable sizes, and (f) is easilyand quickly field installed without the separate use of specialdielectric compounds or fluids or hood lug crimping tools.

Although the present invention is susceptable to various modificationsand alternative constructions, only a preferred embodiment has beenshown in the drawings and described in detail. Such disclosure is notintended to limit the invention. The aim is to cover all modificationsand alternative constructions falling within the spirit and scope of theinvention as expressed in the appended claims.

Various features of the invention are set forth in the following claims.

What is claimed is:

1. A terminator for use with a prepared end of a power cable having anexposed end of a shield layer and an exposed dielectric insulationportion, a part of said exposed dielectric insulation portion adjacentsaid exposed end of the shield being a region of high electrical stress,said terminator comprising a rigid insulator body having an axialpassageway therethrough for application in telescopic relation over saidprepared end of said cable and an inner stress relief element forinterposition between said insulator body and said prepared end of saidcable, said element including a first portion comprising a compressibleelastomer and having electrically conductive properties, said firstportion forming a stress control shield for electrical engagement withsaid shield layer of said cable, and a second portion contiguous to saidfirst portion along an interface between said portions for fitting insurrounding relation and in radial compressive stress over said regionof said prepared cable end, said second portion having higher dielectricstrength than said first portion and comprising a less compressibleelastomer than that of said first portion, said second portion providingdielectric insulation adjacent said first portion, said interfacebetween said first and second portions being directed such that theradial forces in the zone of said element surrounding said region ofsaid prepared cable end intersect parts adjacent one another of bothsaid first and said second portions, such intersected parts of saidsecond portion being disposed inwardly of those of said first portionand in surrounding relation to said region to provide dielectricinsulation in surrounding engagement with said region, and suchintersected parts of said first portion being outwardly adjacent thoseof said second portion and providing compressibility in the zone of saidelement surrounding said region for continuous compliance of saidelement to said cable, said first and second portions of said elementbeing under radial compressive stress relative to the axis of said cablewhen said rigid insulator body is applied to said prepared cable end andsaid element is interposed therebetween.

2. The terminator in accordance with claim 1, wherein said compressibleelastomer of said first portion of said element is of closed-cell spongematerial and said elastomer of said second portion of said element is aclosed-cell elastomer having a density that is greater than the densityof the elastomer of said first portion.

3. The terminator in accordance with claim 1, wherein said compressibleelastomer of said first'portion of said element is of closed-cell spongematerial and said elastomer of said second portion of said element is aclosed-cell elastomer having cells the average size of which is smallerthan the average size of the cells of the elastomer of said firsportion.

4. The terminator in accordance with claim 1, wherein said compressibleelastomer of said first portion of said element is of closed-cell spongematerial and said elastomer of said second portion of said element is asolid elastomer.

5. The terminator in accordance with claim 1, wherein said first andsecond portions of said element are distinct and wherein said elementfurther includes a third distinct portion axially aligned in tandem withrespect to said other portions and comprising a compressible elastomer,the three portions forming a composite unit, said composite unit havinga generally cylindrical outer surface and an axial bore extendingtherethrough, the diameter of said bore when the unit is in anuncompressed state being less than the diameter of said exposeddielectric insulation of said cable, and the outer diameter of saidcomposite unit being essentially equal to the diameter of at least aportion of said axial passageway in said rigid insulator body, wherebysaid composite unit is placed under radial compressive stress relativeto the axis of said cable when said terminator is applied in telescopicrelation to said prepared end of said cable and said composite unit isinterposed therebetween.

6. The terminator in accordance with claim 5, wherein; said firstportion comprises an opening extending axially therethrough, said firstportion opening including a part of said axial bore and a cavityadjoining and axially aligned therewith and increasing outwardly andaway therefrom; said second portion comprises a generally frustoconicalform, one end of which has an outer surface of a shape complementary tosaid first portion cavity to interfit therewith, and an openingextending axially therethrough, said second portion opening including apart of said axial bore and a cavity adjoining and axially alignedtherewith and increasing outwardly and away therefrom; and said thirdportion comprises one end having an outer surface of a shapecomplementary to said second portion cavity to interfit therewith, and apart of said axial bore therein.

7. The terminator in accordance with claim 6, wherein said cavities ofsaid first and second portions terminate their wide ends in an arcuateform and at points inwardly of said generally cylindrical outer surfaceof said composite unit.

8. The terminator in accordance with claim 6, wherein said first portionfurther comprises an axial counterbore extending inwardly from thenon-cavity end thereof, said counterbore having a diameter greater thanthat of said axial bore and less than that of said exposed end of saidcable shield.

9. The terminator in accordance with claim 1, wherein said electricallyconductive properties of said firs portion comprise a conductive coatingapplied to all surfaces thereof, including the surfaces at saidinterface between said first and second portions.

10. The terminator in accordance with claim 1, wherein said electricallyconductive properties of said first portion comprise the insertion ofelectrically conductive particles throughout the elastomer of said firstportion.

11. The terminator in accordance with claim 1, wherein said interfacebetween said first and second portions tapers relative to the cableaxis.

12. The terminator in accordance with claim 1, further comprising asealable compression type electrical connector for making an externalelectrical connection to said terminator, said connector being disposedat one end of said insulator body and comprising a sealable internalchamber for receiving the stranded conductor of said cable and means forforming said external electrical connection.

13. The terminator in accordance with claim 12, wherein said sealablecompression connector further comprises a threaded opening communicatingwith said chamber, a set screw disposed within said threaded opening forturning inwardly against the strands of said cable conductor to compressthe strands against the inner walls of said chamber, and a coating ofsealant disposed between the threads of said set screw and said opening.

14. The terminator in accordance with claim 13, wherein said means forforming said external electrical connection comprises a base on saidconnector, said base having at least a first semicylindrical surfacedisposed therein for receiving an external conductor; a clamp releasablyattached to said base for tightening against said external conductor,said clamp having at least a first semicylindrical surface disposedtherein; and at least a second semicylindrical surface on either saidbase or said clamp, said second semicylindrical surface having a radiusdifferent from either of said first semicylindrical surfaces to providecombinations of cooperating opposing semicylindrical surfaces foraccommodating various sizes of said external conductor.

15. A sealable compression type electrical connector comprising asealable internal chamber for receiving the stranded conductor of apower cable, means for forming an external connection to said conductor,a plurality of threaded openings disposed closely adjacent one anotherin alignment along the axis of said chamber and in communicationtherewith, a corresponding plurality of headless set screws disposedwithin said threaded openings for turning inwardly against the strandsof said conductor to compress the strands against the inner walls ofsaid chamber, and a coating of sealant disposed between the threads ofeach of said set screws and said openings.

16. The connector in accordance with claim 15, wherein said means forforming an external electrical connection comprises a base on saidconnector, said base having at least a first semicylindrical surfacedisposed therein for receiving an external conductor; a clamp releasablyattached to said base for tightening against said external conductor,said clamp having at least a first semicylindrical surface disposedtherein; and at least a second semicylindrical surface on either saidbase or said clamp, said second semicylindrical surface having a radiusdifferent from either of said first semicylindrical surfaces to providea combination of cooperating opposing semicylindrical surfaces foraccommodating various sizes of said external conductor.

17. A stress relief element having a generally cylindrical form with anaxial bore therethrough and comprising: a first portion comprising acompressible elastomer and having electrically conductive properties,said first portion forming a stress control shield; and a second portionin axial alignment with said first portion and contiguous thereto alonga tapering interface between said portions, said interface extendingoutwardly and away from the first portion end of said element, and saidsecond portion having higher dielectric strength than said first portionand comprising a less compressible elastomer than the elastomer of saidfirst portion, said second portion providing dielectric insulationadjacent said first portion.

18. The stress relief element in accordance with claim 17, wherein saidcompressible elastomer of said first portion of said element is ofclosed-cell sponge material and said elastomer of said second portion ofsaid element is a closed-cell elastomer having a density that is greaterthan the density of the elastomer of said first portion.

19. The stress relief element in accordance with claim 17, wherein saidcompressible elastomer of said first portion of said element is ofclosed-cell sponge material and said elastomer of said second portion ofsaid element is a closed-cell elastomer having cells the aver- 18element are distinct and wherein said element further includes a thirddistinct portion axially aligned in tandem with respect to said otherportions and comprising a compressible elastomer, the three portionsforming a composite unit, said composite unit having a generallycylindrical outer surface and an axial passageway extending therethroughUNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 796 rDated March 12 1974 Inventor(s) George E. Lusk It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column l, line 13, "terminator" should be in quotes.

Column 2, line 19, "wet" should be in quotes.

line 57, "dry" should be in quotes.

Column 3, line 7, "dry" should be in quotes.

Column 9, line '31, "vable" should read cable Column 10, line 20, "It isseen" should read It is then seen- Column 14, line 54, "firs" shouldread first Column 15 line 40, "firs" should read first Signed and sealedthis 16th day of July 1971+.

(SEAL) Attest:

MCCOY IYI. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissionerof Patents FORM PC4050 l uscoMM-Dc 60376-P69 U.S. GOVERNMQLI WNTINGOFFICE IQD 0-866-334.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION patent 3,796, 2321Dated March 12, 1974 Inventor(s) George E. Lusk It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1, line 13, "terminator" should be in quotes.

Column 2, line 19 "wet" should be in quotes.

line 57, "dry" should be in quotes.

Column 3, line 7, "dry" should be in quotes.

Column 9, line "vable" should read cable Column 10 line 20, "It is seen"should read It is then seen- Column 14, line 54, "firs" should readfirst Column 15 line 40, "firs" should read first Signed and sealed this16th day of July 1971+.

(SEAL) Attest:

McCOY M. GIBSON, JR. C; MARSHALL DANN Attesting Officer Commissioner ofPatents F ORM PO-105O (10-69)

1. A terminator for use with a prepared end of a power cable having anexposed end of a shield layer and an exposed dielectric insulationportion, a part of said exposed dielectric insulation portion adjacentsaid exposed eNd of the shield being a region of high electrical stress,said terminator comprising a rigid insulator body having an axialpassageway therethrough for application in telescopic relation over saidprepared end of said cable and an inner stress relief element forinterposition between said insulator body and said prepared end of saidcable, said element including a first portion comprising a compressibleelastomer and having electrically conductive properties, said firstportion forming a stress control shield for electrical engagement withsaid shield layer of said cable, and a second portion contiguous to saidfirst portion along an interface between said portions for fitting insurrounding relation and in radial compressive stress over said regionof said prepared cable end, said second portion having higher dielectricstrength than said first portion and comprising a less compressibleelastomer than that of said first portion, said second portion providingdielectric insulation adjacent said first portion, said interfacebetween said first and second portions being directed such that theradial forces in the zone of said element surrounding said region ofsaid prepared cable end intersect parts adjacent one another of bothsaid first and said second portions, such intersected parts of saidsecond portion being disposed inwardly of those of said first portionand in surrounding relation to said region to provide dielectricinsulation in surrounding engagement with said region, and suchintersected parts of said first portion being outwardly adjacent thoseof said second portion and providing compressibility in the zone of saidelement surrounding said region for continuous compliance of saidelement to said cable, said first and second portions of said elementbeing under radial compressive stress relative to the axis of said cablewhen said rigid insulator body is applied to said prepared cable end andsaid element is interposed therebetween.
 2. The terminator in accordancewith claim 1, wherein said compressible elastomer of said first portionof said element is of closed-cell sponge material and said elastomer ofsaid second portion of said element is a closed-cell elastomer having adensity that is greater than the density of the elastomer of said firstportion.
 3. The terminator in accordance with claim 1, wherein saidcompressible elastomer of said first portion of said element is ofclosed-cell sponge material and said elastomer of said second portion ofsaid element is a closed-cell elastomer having cells the average size ofwhich is smaller than the average size of the cells of the elastomer ofsaid firs portion.
 4. The terminator in accordance with claim 1, whereinsaid compressible elastomer of said first portion of said element is ofclosed-cell sponge material and said elastomer of said second portion ofsaid element is a solid elastomer.
 5. The terminator in accordance withclaim 1, wherein said first and second portions of said element aredistinct and wherein said element further includes a third distinctportion axially aligned in tandem with respect to said other portionsand comprising a compressible elastomer, the three portions forming acomposite unit, said composite unit having a generally cylindrical outersurface and an axial bore extending therethrough, the diameter of saidbore when the unit is in an uncompressed state being less than thediameter of said exposed dielectric insulation of said cable, and theouter diameter of said composite unit being essentially equal to thediameter of at least a portion of said axial passageway in said rigidinsulator body, whereby said composite unit is placed under radialcompressive stress relative to the axis of said cable when saidterminator is applied in telescopic relation to said prepared end ofsaid cable and said composite unit is interposed therebetween.
 6. Theterminator in accordance with claim 5, wherein; said first portioncomprises an opening extending axially therethrough, said first Portionopening including a part of said axial bore and a cavity adjoining andaxially aligned therewith and increasing outwardly and away therefrom;said second portion comprises a generally frustoconical form, one end ofwhich has an outer surface of a shape complementary to said firstportion cavity to interfit therewith, and an opening extending axiallytherethrough, said second portion opening including a part of said axialbore and a cavity adjoining and axially aligned therewith and increasingoutwardly and away therefrom; and said third portion comprises one endhaving an outer surface of a shape complementary to said second portioncavity to interfit therewith, and a part of said axial bore therein. 7.The terminator in accordance with claim 6, wherein said cavities of saidfirst and second portions terminate their wide ends in an arcuate formand at points inwardly of said generally cylindrical outer surface ofsaid composite unit.
 8. The terminator in accordance with claim 6,wherein said first portion further comprises an axial counterboreextending inwardly from the non-cavity end thereof, said counterborehaving a diameter greater than that of said axial bore and less thanthat of said exposed end of said cable shield.
 9. The terminator inaccordance with claim 1, wherein said electrically conductive propertiesof said firs portion comprise a conductive coating applied to allsurfaces thereof, including the surfaces at said interface between saidfirst and second portions.
 10. The terminator in accordance with claim1, wherein said electrically conductive properties of said first portioncomprise the insertion of electrically conductive particles throughoutthe elastomer of said first portion.
 11. The terminator in accordancewith claim 1, wherein said interface between said first and secondportions tapers relative to the cable axis.
 12. The terminator inaccordance with claim 1, further comprising a sealable compression typeelectrical connector for making an external electrical connection tosaid terminator, said connector being disposed at one end of saidinsulator body and comprising a sealable internal chamber for receivingthe stranded conductor of said cable and means for forming said externalelectrical connection.
 13. The terminator in accordance with claim 12,wherein said sealable compression connector further comprises a threadedopening communicating with said chamber, a set screw disposed withinsaid threaded opening for turning inwardly against the strands of saidcable conductor to compress the strands against the inner walls of saidchamber, and a coating of sealant disposed between the threads of saidset screw and said opening.
 14. The terminator in accordance with claim13, wherein said means for forming said external electrical connectioncomprises a base on said connector, said base having at least a firstsemicylindrical surface disposed therein for receiving an externalconductor; a clamp releasably attached to said base for tighteningagainst said external conductor, said clamp having at least a firstsemicylindrical surface disposed therein; and at least a secondsemicylindrical surface on either said base or said clamp, said secondsemicylindrical surface having a radius different from either of saidfirst semicylindrical surfaces to provide combinations of cooperatingopposing semicylindrical surfaces for accommodating various sizes ofsaid external conductor.
 15. A sealable compression type electricalconnector comprising a sealable internal chamber for receiving thestranded conductor of a power cable, means for forming an externalconnection to said conductor, a plurality of threaded openings disposedclosely adjacent one another in alignment along the axis of said chamberand in communication therewith, a corresponding plurality of headlessset screws disposed within said threaded openings for turning inwardlyagainst the strands of said conductor to compress the strands againstthe inner walls of said chamber, and a coating of sealant disposedbetween the threads of each of said set screws and said openings. 16.The connector in accordance with claim 15, wherein said means forforming an external electrical connection comprises a base on saidconnector, said base having at least a first semicylindrical surfacedisposed therein for receiving an external conductor; a clamp releasablyattached to said base for tightening against said external conductor,said clamp having at least a first semicylindrical surface disposedtherein; and at least a second semicylindrical surface on either saidbase or said clamp, said second semicylindrical surface having a radiusdifferent from either of said first semicylindrical surfaces to providea combination of cooperating opposing semicylindrical surfaces foraccommodating various sizes of said external conductor.
 17. A stressrelief element having a generally cylindrical form with an axial boretherethrough and comprising: a first portion comprising a compressibleelastomer and having electrically conductive properties, said firstportion forming a stress control shield; and a second portion in axialalignment with said first portion and contiguous thereto along atapering interface between said portions, said interface extendingoutwardly and away from the first portion end of said element, and saidsecond portion having higher dielectric strength than said first portionand comprising a less compressible elastomer than the elastomer of saidfirst portion, said second portion providing dielectric insulationadjacent said first portion.
 18. The stress relief element in accordancewith claim 17, wherein said compressible elastomer of said first portionof said element is of closed-cell sponge material and said elastomer ofsaid second portion of said element is a closed-cell elastomer having adensity that is greater than the density of the elastomer of said firstportion.
 19. The stress relief element in accordance with claim 17,wherein said compressible elastomer of said first portion of saidelement is of closed-cell sponge material and said elastomer of saidsecond portion of said element is a closed-cell elastomer having cellsthe average size of which is smaller than the average size of the cellsof the elastomer of said first portion.
 20. The stress relief element inaccordance with claim 17, wherein said compressible elastomer of saidfirst portion of said element is of closed-cell sponge material and saidelastomer of said second portion of said element is a solid elastomer.21. The stress relief element in accordance with claim 17, wherein saidfirst and second portions of said element are distinct and wherein saidelement further includes a third distinct portion axially aligned intandem with respect to said other portions and comprising a compressibleelastomer, the three portions forming a composite unit, said compositeunit having a generally cylindrical outer surface and an axialpassageway extending therethrough.